Milling devices, such as ball mills or roller mills are typically used to grind or pulverize material to a smaller size. Typically, the mills are configured to grind material to a size within a predetermined size range. Milling devices are often used in crushing circuits for mining operations, or for crushing or grinding material for use in cement manufacturing. Examples of milling devices may be found in U.S. Pat. Nos. 1,585,755, 2,115,314, 3,269,668, 3,337,142, 3,497,3214,886,216, 4,934,613, 5,100,068, 5,355,707, 5,361,996, 5,405,091, 5,716,015, 6,318,649, 6,508,421, 7,028,934 or U.S. Patent Application Publication Nos. 2002/0011534, 2002/0139880 or 2005/0045756.
Some types of milling devices include roll presses, roller mills or ball mills. Roll presses generally compress material between two rollers. Roll presses often have a hydraulic system configured to hold the rollers together to apply the compression to the material being ground, or milled. Roll presses are typically difficult to control material grinding size and, as a result, are generally less effective at making very finely ground material compared to other milling devices.
Roller mills typically have vertical wheels that push down on a driven table, or conveyor. Hydraulic cylinders or other mechanism are often used to push the vertical wheels or rollers, down on the driven table or conveyor. Roller mills typically directly apply a combination of compression and shear forces to grind material. This direct contact results in efficient grinding. Table roller mills typically feature a gear reducer that usually has large thrust capabilities to support the table and the grinding force of the mill. A roller mill typically has its grinding force limited by the gear reducer's upward thrust capability. Moreover, the grinding forces are typically required to pass through roller bearings that transmit force through line contact. Roller mills often have complicated stands, pressure levers, lubrication systems, multistage gear reducers, and usually require relatively large bearings. The cost of roller mills is generally high relative to other mills, such as ball mills.
Ball mills typically have a cylinder full of spherical balls that rotates with raw material floating in between the balls within the cylinder. The balls are lifted by the spinning action of the cylinder and fall or cascade on one another to grind material via impact and shearing forces. Ball mills are typically simple in design. However, operation of ball mills usually uses more energy relative to other types of mills. For instance, the random nature of the ball movement sometimes results in the balls hitting each other instead of the material, which wastes energy.
A new type of mill is needed that provides for a simple mill design that can be made and sold at a relatively low cost and also efficiently utilizes energy and provides compression and shearing forces that act on material to grind that material. Preferably, such a mill would be designed to be less expensive than roller mills and more energy efficient than ball mills.